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Author Topic: DSP amplifier limiter calculation  (Read 6598 times)

Roberto Orozco

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DSP amplifier limiter calculation
« on: February 19, 2018, 08:17:39 PM »

hi, I hope somebody can help me with this issue. I recently bought an amplifier module for a pair of speaker cabinets I'm  building. the amplifier comes with DSP features, but is not programmed yet and I'm not sure how to do this. I contacted the manufacturer for help but they just sent me a link to download the software to adjust the different settings of the module. they told me is really easy!  it may be!  for somebody with the knowledge in the field.

I'm not a sound engineer, I been doing the sound with my band (small town band) for some years, we use an analogue mixer to connect all the signals, mixer to crossover, crossover to amps with no DSP, amps to speakers.
I been doing some research about this and looks like the DSP settings of an amplifier are base on the components that I'm going to be using in my cabinets and their specifications.

the amplifier has 3-channels
1500watts-low    500watts-mid    500watts-highs------at 8ohms
1800                  900                  900                 ------at 4ohms
RMS power.

the amp is to power 2 cabinets, each cabinet includes:

1 X 12'' subwoofer  550watts RMS  8ohms
1 X 6''   driver     100watts RMS   8ohms
1 X 1.75''  high frequency driver  100watts RMS  8ohms

if somebody could tell me how to setup the DSP features on the amplifier to use it with this speakers please!

I understand that to calculate the different settings, gain, limiters, crossover points, of the amplifier I need to know the specs of the speakers and the amp. I'll put the pictures attached with the information that the manufacturer gave me for the amp and the information I have from the speakers, I hope this helps.

I been trying to build my own speaker cabinets since long time ago, I hope I can find the help to complete my project  in this forum.

thank you so much for looking.     
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Chris Grimshaw

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Re: DSP amplifier limiter calculation
« Reply #1 on: February 20, 2018, 03:54:34 AM »

Here's how I'd do it:

Limiters:
Sub - set a fairly long attack time (couple of seconds) and an even longer release time at -3dB.
Mid - 0.5s attack, 2s release, -6dB
High - fast attack and release, -6dB

Those will kick in if you hit the drivers with too much power and cut the signal down to something the drivers can manage thermally.

Dial in whatever crossover points the manufacturer states. I'd use 4th order Linkwitz-Riley on all of them*. Make sure you set a highpass filter on the 12" to cut off the very low bass. A poof of air into a microphone will launch cones across the room without that filter.

When you've done all that, play a test tone at the crossover frequency. Set it so the midrange driver has reversed polarity. Now, play with the delay on the midrange driver until you hear the minimum amount of sound. If you increase delay, you'll find that you can make that happen many times. You want to use the minimum delay required to still get that cancellation. Set the midrange driver for correct polarity and you should have lots of sound again.
Repeat this for the high-frequency driver. Reversed polarity for HF, play with delays, etc.

That should get your system somewhere near time-aligned. It's not the best method, but if you don't have a measurement microphone etc it's a pretty good way of doing it. It can be quite difficult with the high-frequency section - I'd recommend doing it outside so there aren't any reflections messing things up.

Play with the low/mid/high levels until you're happy with the system balance, and you're done.

*I like LR4 because it offers a decent amount of driver protection, and the two bands are electrically in-phase at the crossover point, which makes finding the delays much easier.

Chris
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Roberto Orozco

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Re: DSP amplifier limiter calculation
« Reply #2 on: February 20, 2018, 12:23:11 PM »

thank you so much Chris for taking time to give me this information. I appreciate that!  if you don't mind, can you tell me how you calculate those numbers for the limiter?
I was trying to do it myself using amplifier limiter calculators on the internet but I couldn't do it because I was missing the gain voltage of the amp in dB. or some other number.

I think I can do what you say with the delay, the part that i'm not completely sure is how to reversed the polarity  to test the drivers.
excuse me if I can't understand completely what you are explaining to me, maybe is not really difficult but honestly I never had to dig inside of the DSP settings of an amplifier before.
the pictures that I attached are from the software itself, those are the screens that I'm going to be working on and there are numbers and boxes and numbers that I don't understand very well... but I would like to.  I'm not asking you to teach me all about DSP programming I just hope you be patient and if you have the time, help me to understand a little more what I'm trying to do.

I'll really appreciate your help and your time to replay. thank you.     
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Mike Caldwell

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Re: DSP amplifier limiter calculation
« Reply #3 on: February 20, 2018, 04:49:42 PM »

Here's how I'd do it:

Limiters:
Sub - set a fairly long attack time (couple of seconds) and an even longer release time at -3dB.
Mid - 0.5s attack, 2s release, -6dB
High - fast attack and release, -6dB

Those will kick in if you hit the drivers with too much power and cut the signal down to something the drivers can manage thermally.

Dial in whatever crossover points the manufacturer states. I'd use 4th order Linkwitz-Riley on all of them*. Make sure you set a highpass filter on the 12" to cut off the very low bass. A poof of air into a microphone will launch cones across the room without that filter.

When you've done all that, play a test tone at the crossover frequency. Set it so the midrange driver has reversed polarity. Now, play with the delay on the midrange driver until you hear the minimum amount of sound. If you increase delay, you'll find that you can make that happen many times. You want to use the minimum delay required to still get that cancellation. Set the midrange driver for correct polarity and you should have lots of sound again.
Repeat this for the high-frequency driver. Reversed polarity for HF, play with delays, etc.

That should get your system somewhere near time-aligned. It's not the best method, but if you don't have a measurement microphone etc it's a pretty good way of doing it. It can be quite difficult with the high-frequency section - I'd recommend doing it outside so there aren't any reflections messing things up.

Play with the low/mid/high levels until you're happy with the system balance, and you're done.

*I like LR4 because it offers a decent amount of driver protection, and the two bands are electrically in-phase at the crossover point, which makes finding the delays much easier.

Chris

I did that same thing and went with what I felt was the deepest null point and then later checked it with some measurement software that would display the magnitude and phase response and I found that most of my by "ear" delay settings were fairly close.

Paul G. OBrien

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Re: DSP amplifier limiter calculation
« Reply #4 on: February 20, 2018, 11:35:57 PM »

the part that i'm not completely sure is how to reversed the polarity  to test the drivers.
Roberto... in the third pic down in your first post there appear to be buttons in the flow diagram that are labelled POL, I believe that is short for Polarity. I bet if you click on that you will see options for normal(+) or inverted(-).
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Roberto Orozco

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Re: DSP amplifier limiter calculation
« Reply #5 on: February 21, 2018, 12:35:44 PM »

Roberto... in the third pic down in your first post there appear to be buttons in the flow diagram that are labelled POL, I believe that is short for Polarity. I bet if you click on that you will see options for normal(+) or inverted(-).

thank you so much Paul, I'll get to work on it. 
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Chris Grimshaw

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Re: DSP amplifier limiter calculation
« Reply #6 on: February 21, 2018, 01:23:50 PM »

thank you so much Chris for taking time to give me this information. I appreciate that!  if you don't mind, can you tell me how you calculate those numbers for the limiter?   

I made the assumption (rightly or wrongly) that 0dB would be maximum power, and therefore -3dB would be half-power.
The time constants are just educated estimations of the thermal masses of the voicecoils in use for each band. You could try asking the manufacturer if they've got any recommended limiter settings for each driver, but I wouldn't bet on an answer.

Chris
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Roberto Orozco

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Re: DSP amplifier limiter calculation
« Reply #7 on: February 21, 2018, 03:35:52 PM »

I made the assumption (rightly or wrongly) that 0dB would be maximum power, and therefore -3dB would be half-power.
The time constants are just educated estimations of the thermal masses of the voicecoils in use for each band. You could try asking the manufacturer if they've got any recommended limiter settings for each driver, but I wouldn't bet on an answer.

Chris
 
Hi Chris thanks again for your help.  well, like I mention before, I was trying to calculate the limiter for the amp using a limiter calculator, I don't remember the website, but they ask me for the power/voltage gain of the amplifier in decibels in order to do the calculation. I contacted the manufacturer letting them know what I was trying to do and this was the answer I get:

0dB for LF;  -10dB  for MF;  -12dB for HF.

exactly like that.
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Chris Grimshaw

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Re: DSP amplifier limiter calculation
« Reply #8 on: February 22, 2018, 03:41:29 AM »


Hi Chris thanks again for your help.  well, like I mention before, I was trying to calculate the limiter for the amp using a limiter calculator, I don't remember the website, but they ask me for the power/voltage gain of the amplifier in decibels in order to do the calculation. I contacted the manufacturer letting them know what I was trying to do and this was the answer I get:

0dB for LF;  -10dB  for MF;  -12dB for HF.

exactly like that.

The calculator you were using needs to consider how much gain the amplifier has in order to work out your limiters - if you limit both to -3dB, but one has 36dB gain and the other has 46dB, then one will be putting out 10dB more signal.

With your amplifier, we're looking at the limiting compared to the maximum power it can give out.

In my opinion, setting for 0dB on the LF would be risky, as the amplifier does have enough power to burn out the woofers if you hit them with a high-power signal with a long duration. Like low-frequency feedback, for instance. The limiting I suggested (with a long attack/release time) won't activate unless there's been a high-power situation for a few seconds, in which case the power levels need to be decreased in order to save the woofers.
Their MF/HF settings are more conservative than mine. I can see why they've done that. It's going a bit safer, but losing a bit of peak output. It's all tradeoffs.

Some amplifiers have RMS limiting, which is really useful - you can tell them the peak/continuous power handling of your drivers, and the amplifier will look after the rest.

Chris
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Ivan Beaver

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Re: DSP amplifier limiter calculation
« Reply #9 on: February 22, 2018, 07:31:34 AM »

The BEST way to set limiter voltages is to MEASURE them, using a sine wave a voltmeter.

Calculators can be useful-IF everything is correct that is entered into them.

I have know some amp gains to be different (as much as 4dB) than what the spec sheet says.

I know one major amp manufacturer who considers the gain to be normal if it is +/- 1 dB over published specs.

Simply put a sine wave in, adjust the gain so that the measured voltage on the output is higher (I like at least 10%) than the voltage you will limit to.

BE SURE the speakers are NOT hooked up at this time!!!!!!!

Then simply turn down the threshold until you get to the desired voltage.

If you are using multistage limiters, be SURE to do the peak or highest limiter first, and the continuous or heating limiter last.
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Re: DSP amplifier limiter calculation
« Reply #9 on: February 22, 2018, 07:31:34 AM »


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